1. Field of the Invention
This invention relates to a method of and an apparatus for taking a plurality of radiation images at different distances from the object and generating a phase contrast image on the basis of the radiation images obtained.
2. Description of the Related Art
There has been known a radiation image reproduction system in which an object is exposed to a radiation (X-rays, α-rays, β-rays, electron beams, ultraviolet rays or the like), the radiation passing through the object is detected by the use of, for instance, a stimulable phosphor sheet (to be described later) or a radiation detector panel (to be described later), thereby obtaining a radiation image data representing a radiation image of the object, and a radiation image is reproduced on the basis of the radiation image data after it is variously processed.
When certain kinds of phosphor are exposed to a radiation (X-rays, α-rays, β-rays, electron beams, ultraviolet rays), they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. Further, a recording sheet comprising a layer of such a stimulable phosphor is referred to as “a stimulable phosphor sheet”. When the stimulable phosphor sheet is used, the stimulable phosphor sheet is exposed to stimulating light after exposed to a radiation passing through an object and the stimulated emission emitted from the stimulable phosphor sheet upon exposure to the stimulating light is photoelectrically read, thereby obtaining image data representing a radiation image of the object. The radiation detector panel comprises a plurality of two-dimensionally arranged detecting elements and the detecting elements generates electric signals proportional to the amount of radiation projected onto the panel. Image data representing a radiation image of the object is obtained on the basis of the electric signals output from the detecting elements.
The radiation image thus obtained represents difference in intensity of the radiation passing through the object. For example, when the object includes a bone and a soft tissue, the radiation passing through the bone is largely attenuated and a very small part of the radiation reaches the detector (e.g., a stimulable phosphor sheet or a radiation detector panel) whereas the radiation passing through the soft tissue is less attenuated and a relatively large part of the radiation reaches the detector. Accordingly, in the case of such an object, the bone is expressed in white and the soft tissue is expressed in black. That is, a radiation image obtained is large in contrast and rich in information.
However, when the object mainly includes only soft tissues like a mammogram, difference in radiation attenuation by tissues is not so large, and accordingly, a radiation image obtained is small in contrast and poor in information.
In order to overcome this problem, there has been proposed a phase contrast imaging in which phase difference of radiation generated when the radiation passes through the object is visualized. The phase contrast imaging is based on the fact that when radiation is projected onto different materials, the phase of the wave of the radiation changes before and after passing through the materials and a phase difference is generated due to difference in propagation in the materials since radiation is an electromagnetic wave like light. When the object is of a soft part, a fine difference in tissues included in the soft part can be more clearly visualized by the phase contrast imaging since the phase difference is larger than the difference in attenuation. The phase contrast imaging is described in detail, for instance, in “Quantitative aspects of coherent hard X-ray imaging: Talbot images and holographic reconstruction” by Peter Cloetens, et al., (Proc, SPIE, Vol. 3154(1997), 72-82) (will be referred to as “paper 1”, hereinbelow), and “Hard x-ray phase imaging using simple propagation of a coherent synchrotron radiation beam” by Peter Cloetens, et al., J. Phys. D:Appl. Phys.32(1999), A145-A151 (will be referred to as “paper 2”, hereinbelow). According to these papers, a phase contrast image can be generated by taking images at a plurality of distances from the object by the use of a two-dimensional sensor (e.g., a radiation detector panel), thereby obtaining a plurality of pieces of image data representing a plurality of radiation images, and carrying out operation based on a predetermined algorithm by the use of the plurality of pieces of image data.
When taking a radiation image, the amount of radiation impinging upon a two-dimensional detector changes in inverse proportion to the square of the distance between the radiation source and the detector. Further sine the radiation is emitted from the radiation source to diverge away from the radiation source, the size of the radiation image as detected by the two-dimensional detector increases as the distance between the detector and the radiation source increases. Accordingly, there have been proposed radiation image taking apparatuses in which the gain at which the radiation image is read is controlled according to the distance between the object and the two-dimensional detector or the rate of enlargement is obtained. See, for instance, Japanese Unexamined Patent Publication No. 2000-245721.
The radiation projected onto the object in the phase contrast imaging is slightly divergent though it is substantially parallel light and accordingly, the sizes of the radiation images become slightly larger as the distance from the object increases. Accordingly, when a phase contrast image is generated on the basis of the images as taken in different imaging positions, position shift occurs due to mismatch of the sizes of the radiation images, which results in errors in the phase contrast image.
The amount of radiation impinging upon a two-dimensional detector is reduced as the distance from the object increases. Further, in the case, where a plurality of radiation images are obtained with a plurality of two-dimensional detectors disposed in a plurality of imaging positions, the amount of radiation impinging upon a two-dimensional detector is reduced also depending upon the number of detectors which the radiation passes through before impinging upon the detector. Accordingly, when a phase contrast image is generated on the basis of the images as taken in different imaging positions, it becomes difficult to precisely generate a phase contrast image due to mismatch of densities of the radiation images.